Crosstalk (or inter-channel interference) is a major source of channel impairment for Multiple Input Multiple Output (MIMO) wired communication systems, such as Digital Subscriber Line (DSL) communication systems.
As the demand for higher data rates increases, DSL systems are evolving toward higher frequency bands, wherein crosstalk between neighboring transmission lines (that is to say transmission lines that are in close vicinity over part or whole of their length, such as twisted copper pairs in a cable binder) is more pronounced (the higher frequency, the more coupling).
Different strategies have been developed to mitigate crosstalk and to maximize effective throughput, reach and line stability. These techniques are gradually evolving from static or dynamic spectral management techniques to multi-user signal coordination (vectoring herein after).
One technique for reducing inter-channel interference is joint signal precoding: the transmit data symbols are jointly passed through a precoder before being transmitted over the respective communication channels. The precoder is such that the concatenation of the precoder and the communication channels results in little or no inter-channel interference at the receivers.
A further technique for reducing inter-channel interference is joint signal post-processing: the receive data symbols are jointly passed through a postcoder before being detected. The postcoder is such that the concatenation of the communication channels and the postcoder results in little or no inter-channel interference at the receivers.
More formally, a vectored system can be described by the following linear model:Y(k)=H(k)X(k)+Z(k)  (1),wherein the N-component complex vector X, respectively Y, denotes a discrete frequency representation, as a function of the carrier index k, of the symbols transmitted over, respectively received from, the N vectored channels,wherein the N×N complex matrix H is referred to as the channel matrix: the (n,m)-th component Hnm of the channel matrix H describes how the communication system produces a signal on the n-th channel output in response to a signal being transmitted to the m-th channel input; the diagonal elements of the channel matrix describe direct channel coupling, and the off-diagonal elements of the channel matrix (also referred to as the crosstalk coefficients) describe inter-channel coupling,and wherein the N-component complex vector Z denotes additive noise over the N channels, such as Radio Frequency Interference (RFI) or thermal noise.
Linear signal precoding and post-processing are advantageously implemented by means of matrix products.
In downstream, the linear precoder performs a matrix-product in the frequency domain of a transmit vector U(k) with a precoding matrix P(k), i.e. X(k)=P(k)U(k) in eq. (1), the precoding matrix P(k) being such that the overall channel matrix H(k)P(k) is diagonalized, meaning the off-diagonal coefficients of the overall channel H(k)P(k), and thus the inter-channel interference, reduce to almost zero.
Practically, and as a first order approximation, the precoder superimposes anti-phase crosstalk pre-compensation signals over the victim line along with the direct signal that destructively interfere at the receiver with the actual crosstalk signals from the respective disturber lines.
In upstream, the linear postcoder performs a matrix-product in the frequency domain of the receive vector Y(k) with a crosstalk cancellation matrix Q(k) to recover the transmit vector U(k) (after channel equalization and power normalization), the crosstalk cancellation matrix Q(k) being such that the overall channel matrix Q(k)H(k) is diagonalized, meaning the off-diagonal coefficients of the overall channel Q(k)H(k), and thus the inter-channel interference, reduce to almost zero.
Signal vectoring is typically performed within an access node, wherein all the data symbols concurrently transmitted over, or received from, all the subscriber lines are available. For instance, signal vectoring is advantageously performed within a Digital Subscriber Line Access Multiplexer (DSLAM) deployed at a Central Office (CO) or as a fiber-fed remote unit closer to subscriber premises (street cabinet, pole cabinet, etc). Signal precoding is particularly appropriate for downstream communication (toward customer premises), while signal post-processing is particularly appropriate for upstream communication (from customer premises).
The choice of the vectoring group, that is to say the set of communication lines, the signals of which are jointly processed, is rather critical for achieving good crosstalk mitigation performances. Within a vectoring group, each communication line is considered as a disturber line inducing crosstalk into the other communication lines of the group, and the same communication line is considered as a victim line receiving crosstalk from the other communication lines of the group. Crosstalk from lines that do not belong to the vectoring group is treated as alien noise and is not canceled.
Ideally, the vectoring group should match the whole set of communication lines that physically and noticeably interact with each other. Yet, legal or technical restrictions may prevent such an exhaustive approach, in which case the vectoring group would include a sub-set only of all the physically interacting lines, thereby yielding limited vectoring gains.
For instance, regulators in certain countries require to have Sub-Loop Unbundling (SLU), whereby a new telecommunication provider company known as a Competitive Local Exchange Carrier (CLEC) gets granted a physical access to the copper plant, and is allowed to install its own network equipment alongside the network equipment of the Incumbent Local Exchange Carrier (ILEC). In this deployment model, the lines of different operators typically share the same cable or cable binder. As the lines are connected to different network equipment that are not coordinated, the resulting vectoring gains are reduced and can be as low as 5 to 10% depending on the crosstalk levels of the “alien” disturbers.
In some countries, SLU has been omitted in case of vectoring deployments. Instead the ILEC or any designated operator supplies an access to the subscriber's individual Layer 2 (L2) or Layer 3 (L3) bit streams at one or more central aggregation points. The other operators connect to the aggregation points and pick up the relevant bit streams from their respective subscribers.
A second option could be to enable “cross-DSLAM” vectoring while allowing operators to use their own equipment suppliers. Although, this could be feasible from a theoretical and technical point of view, it means that non-standard vectoring interfaces and algorithms need to be agreed upon between competitors, which makes the solution practically infeasible.